1. Field of the Invention
This invention relates to dual wavelength spectrophotometers.
2. Description of Related Art
There are two primary methods for generating dual wavelength light beams for dual wavelength spectrophotometers. Those techniques are illustrated in FIGS. 1 and 2 labeled "Prior Art".
FIG. 1 illustrates a known technique in which two light sources L.sub.1 and L.sub.2 provide illumination respectively for monochromators M.sub.1 and M.sub.2 which are set at different wavelengths. The output beams of monochromators M.sub.1 and M.sub.2 are therefore at different wavelengths. A chopper C or optical modulator is then used to alternately allow the two different wavelengths to combine and illuminate a single sample. One of the major problems with the prior art technique illustrated in FIG. 1 is that the intensities of lamps L.sub.1 and L.sub.2 can never be perfectly matched. This results in a variation in illumination to the sample which can result in undesirable testing error.
FIG. 2 illustrates another known prior art technique which employs a single light source L. The multi-chromatic light from source L is divided by beam splitter BS and focused into two monochromators M.sub.1 and M.sub.2. An alternative embodiment of the technique illustrated in FIG. 2 is to position the two monochromators M.sub.1 and M.sub.2 to intercept light from the same source L, but at different sections of the illumination by positioning the monochromators at a 90 degree or 180 degree angle with respect to each other, thereby eliminating the need for a beam splitter. After the monochromators M.sub.1 or M.sub.2 select two different wavelengths, the outputs are combined by a chopper C or optical modulator causing the two beams to merge into a single beam alternating between the two wavelengths. One of the problems associated with the technique illustrated in FIG. 2 is that the beam splitter BS necessarily decreases the level of illumination available to monochromators M.sub.1 and M.sub.2. Therefore the level of illumination will be only approximately one-half of that available from individual light sources such as L.sub.1 and L.sub.2 shown in FIG. 1. Moreover, the beam splitter BS is not necessarily a perfect beam splitter. Therefore one direction might pass slightly more than 50% of the light beam and the other direction reflect slightly less than 50% of the light beam resulting in erroneous intensity readings at the output of the instrument. Similarly, positioning two monochromators at different angles (e.g. 90 or 180 degrees) with respect to the light source L may also result in unevenness due to the fact that the light is not a perfect light source and its intensity may vary from angle to angle and from time to time. Prior art dual wavelength techniques such as illustrated in FIG. 2 are also described in U.S. Pats. Nos. 3,676,005; 3,666,362 and 4,136,959.
The light source employed with dual wavelength spectrophotometers is an important consideration. A typical prior art light source is illustrated in FIG. 3. The lamp shown in FIG. 3 is positioned in front of a spherical mirror M to collect the light from behind the back of the lamp. A very fast lens L.sub.1, such as an f.7 with a large diameter, for example four inches, is used to collect as much of the light as possible from the back collecting mirror and the front of the lamp. A second lens L.sub.2 is necessary to slow the f number of the beam to that of the monochromator, which generally has an f number of 3.6 or slower.
The use of optical choppers in the context of dual beam or dual wavelength spectrophotometers is known. Many different types of choppers have been employed including vibrating shutters, vibrating gates, rotating mirrors, etc. The use of a partially mirrored, partially transparent optical chopper is disclosed in a number of prior art patents, including, but not limited to the following: U.S. Pat. Nos. 3,039,353; 3,658,422; 3,666,362; 3,676,005; 3,897,154; 4,305,663; 4,455,097 and 4,484,815.
Many systems employ lenses and other refractive optical elements. A few systems are known to use reflective optics in other contexts. See for example, U.S. Pat. No. 3,029,253 which discloses a dual beam spectrophotometer using reflective surfaces.
Insofar as understood, none of the prior art taken singularly or in combination suggests a dual wavelength spectrophotometer having substantially increased spot illumination and substantially reduced dead time as set forth in this disclosure.